Spearmint (Mentha spicata L.) is one of the most important mint family that has numerous applications in food, cosmetical, and pharmaceutical industries due to its valuable medicinal properties. The essential oil content and composition are affected by genotype, climatic, edafic condition and agricultural practices. In this research, the effect of harvest time on plant fresh and leaf dry weights, essential oil content, yield and composition as randomized complete block design with twelve treatments and three replications was studied. Result showed that harvest time had a significant effect on plant fresh and leaf dry weights, essential oil content and yield. The highest plant fresh (2507.3 gr.m-1) and leaf dry (452.2 gr.m-1) weights recorded in July had no significant difference with that of June. Plants also showed the highest amount of essential oil content (2.75%) and yield (6.99 gr.m-1) in September and July, respectively. The main essential oil components were carvone, limonene, careen, α-pinene, myrecene, β-borbonene, cisdihydrocarvone, dihydrocarveol, dihydrocarvyl acetate, Pulgone and trans caryophylene. The highest (57.46%) and lowest (16.83%) amounts of carvone were observed in October and February, respectively. The highest (32.62%) and lowest (2.19%) amounts of limonene were observed in September and March, respectively. The maximum amount of careen (2.87%) was obtained in November. Cis dihydrocarvone, dihidrocarveol and dihydrocarvyl acetate were increased during winter. Overall, in regard to essential oil yield and carvone content, the best time to harvest is from June to September.